Plant structural complexity and host-finding by a parasitoid

Summary There are three major components to plant structure relevant to searching parasitoids: 1) plant size or surface area, 2) the variation among plant parts (structural heterogeneity), such as seed heads, flowers and nectaries, and heterogeneous surfaces (e.g. glabrous, hirsute), and 3) the connectivity of parts or plant form (structural complexity). We examined the effect of structural complexity, while controlling for size and structural heterogeneity, on searching behaviors of Trichogramma nubilale in controlled environments. Females were presented with a structurally simple surface and a structurally complex one. Parasitism rates were 2.9 times higher on simple surfaces than on complex ones. Unexpectedly, when no hosts were present, searching time on simple surfaces was 1.2 times higher than on complex surfaces. This implies that structural complexity per se can affect the giving-up-time of a searching parasitoid. Searching efficiency, however, was the dominant process, and females found hosts on simple surfaces 2.4 times faster than on complex surfaces. Structural complexity can have a dramatic effect on the success of parasitoid search.     

Underestimation of mutual interference of predators

Summary The usual method of estimating the mutual interference constant, m, assumes a linear (type I) functional response of predators. In the cases where the response is not linear, the application of the method introduces a bias in the estimation of the searching efficiencies. It is shown that, as a consequence, the value of m is underestimated. A new method is proposed, which allows for a type II functional response due to a handling time. A comparative analysis of 15 data sets from the literature shows that the proposed method gives values of m that are consistently higher than those estimated by the traditional method. The new method calculates the parameters with nonlinear regression and provides standard errors for the estimates. Therefore, the reliability of the searching efficiencies, the handling time and the constant m can be quantified. Very few of the interference constants are significantly different from m=1. This special value implies that the functional response is a function of the ratio of prey and predator densities. These empirical findings support the suggestion of Arditi and Ginzburg (1989) that the functional response might often be ratio-dependent, especially in complex and heterogeneous situations.     

Evolutionary consequences of a search image

Many predators are able to become better at spotting cryptic prey by recognising specific clues, but by concentrating on one prey type they will become worse at spotting other prey types. This phenomenon is known as the formation of a search image for a certain prey by a predator and is related to apostatic selection. Here, we study the evolution of a search image in the predator by formulating and analysing a mathematical model. The predator forages for two prey types and is able to form an independent search image for both prey. The results show that the evolutionary dynamics can be divided into two parts: a fast and a slow part. At first selection pressure will be strong towards a stable ratio of prey, which is the same as the ratio found for the unbeatable prey choice for predators with a Holling type II functional response. Following this, the slow dynamics will keep this ratio constant independent of the trait values, but the predator will slowly evolve towards a stronger search image and ultimately become a specialist predator or slowly evolve towards generalist with a weak search image. In conclusion, the formation of a search image causes the predator to control the prey densities such that the ratio of available prey is kept constant by the predator.     

Experimental simulation in behavioral ecology: a multimedia approach with the spatial searching simulation RattleSnake?

Searchers in nature often have accurate knowledge of the spatial location of the resource targets they seek, though in many other cases they have none. For example, the spatial distribution of targets such as food patches or potential mates may shift or change unpredictably from season to season. Searchers encountering circumstances of these sorts may be said to be “naive”. This problem is compounded by the fact that spatial distributions of targets may vary statistically as well: they may be distributed randomly, uniformly, or they may be clustered. Accordingly, since we study an animal system in nature that encounters such challenges (i.e., free-ranging rattlesnakes in many parts of their range), we wrote a comprehensive spatial searching program for Macintosh systems that simulates this problem thoroughly, RattleSnake^©. In a large series of experimental simulations using this software, we found that search paths of high vector magnitude (approaching 1.0), or those that approached straight lines, generated large numbers of collisions in large, clustered worlds. No search path was any better than any other in large, randomly or uniformly distributed worlds. Zig-zag paths of low vector magnitude (approaching zero) in small worlds of all types and of all densities were efficacious, due to continuous turning which prevented searchers from moving out of or exiting patches. Thus it appears that there are design rules in nature governing target collision probabilities in some but not all two-dimensional spatial worlds. Search paths of high vector magnitude, or those approaching straight lines, generate high collision frequencies in statistically clustered spatial worlds, for example. RattleSnake^© thus may be useful in programs of basic and/or applied behavioral ecology, including conservation, as well as in laboratory and multimedia classroom education.Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Polyandry and alternative mating tactics

Many species in the animal kingdom are characterized by alternative mating tactics (AMTs) within a sex. In males, such tactics include mate guarding versus sneaking behaviours, or territorial versus female mimicry. Although AMTs can occur in either sex, they have been most commonly described in males. This sex bias may, in part, reflect the increased opportunity for sexual selection that typically exists in males, which can result in a higher probability that AMTs evolve in that sex. Consequently, females and polyandry can play a pivotal role in governing the reproductive success associated with male AMTs and in the evolutionary dynamics of the tactics. In this review, we discuss polyandry and the evolution of AMTs. First, we define AMTs and review game theoretical and quantitative genetic approaches used to model their evolution. Second, we review several examples of AMTs, highlighting the roles that genes and environment play in phenotype expression and development of the tactics, as well as empirical approaches to differentiating among the mechanisms. Third, ecological and genetic constraints to the evolution of AMTs are discussed. Fourth, we speculate on why female AMTs are less reported on in the literature than male tactics. Fifth, we examine the effects of AMTs on breeding outcomes and female fitness, and as a source, and possibly also a consequence, of sexual conflict. We conclude by suggesting a new model for the evolution of AMTs that incorporates both environmental and genetic effects, and discuss some future avenues of research.